The present invention relates to an injection molding machine having an improved system for driving a rotatable member such as a turret or a turntable.
Over the years, a number of different systems have been developed to drive rotatable components in an injection molding machine. For example, it is known to use belt driven systems to drive a rotatable turntable mounted on the moving platen of a horizontal injection molding machine. It also is known to use a belt driven system for driving a turntable mounted on a vertical injection molding machine. This latter type of drive system is illustrated in co-pending U.S. patent application Ser. No. 08/140,424, entitled Vertical Injection Molding Machine, to Elward et al., which is assigned to the assignee of the instant application.
Electric motors are well known in the art and also have been used to drive a number of different components in an injection molding machine. For example, it is known to use an electric motor and gearbox arrangement to rotate mold segments in an injection blow molding machine. Such a system is illustrated in U.S. Pat. No. 5,531,580 to Bonino et al. It is also known in the prior art to use an electric motor to drive via a gearbox or belt the turret block of an injection molding machine to cause rotation of same. Such a system is illustrated in U.S. Pat. No. 5,728,409, entitled Turret Article Molding Machine, to Schad et al., which patent is assigned to the assignee of the instant application.
The principal disadvantage of these systems however is that the drive train, belt or gears, cause inaccuracies in the positioning of the member being driven after rotation. Accurate positioning of a turret for example is required so that mold segments are aligned to allow closing without damage or wear. This disadvantage originates in drive transmission systems which have a degree of elasticity in the transmission of motion. This is due either to clearances in the mechanism (gears) or elasticity in the belt. This lack of close-coupling also reduces the speed of response of the drive since inertial forces are absorbed by the elasticity of the transmission. Quickly stopping or starting such a drive does not cause the turret block to move in complete synchronization. This can be detrimental when optimizing cycle time of the machine.
Electric motors have also been used to drive other components in injection molding machines. For example, electric drives for clamp actuation via intermediate transmissions are shown in U.S. Pat. No. 5,585,126 to Heindl et al.; U.S. Pat. No. 5,565,224 to Stillhard; U.S. Pat. No. 5,513,977 to Kitajima; U.S. Pat. No. 5,338,174 to Miese et al.; U.S. Pat. No. 4,828,475 to Kamiguchi; and U.S. Pat. No. 4,797,086 to Adachi. An electric motor drive for linearly moving an injection screw is shown in U.S. Pat. No. 4,895,505 to Inaba et al. In the Inaba et al. system, a separate screw rotating motor is provided to rotate a shaft connected to the injection screw.
U.S. Pat. No. 5,052,909 to Hertzer et al. illustrates an injection molding machine having a pump driven by a variable speed motor preferably of the brushless DC type. U.S. Pat. No. 4,988,273 to Faig et al. illustrates still another injection molding machine having brushless DC motors for driving the clamping unit, the injection unit, the extruder screw, and the ejector unit.
There remains a need however for a more efficient drive system which does not suffer from the disadvantages of the aforementioned systems.